unable to test a method which uses Observable in Jasmine - angular6

Question

My Angular application has a service QuestionManagementService which response on a BackendService to send REST messages. The BackendService in turn uses HttpClient. I am trying to test the QuestionManagementService in isolation.
The method I am testing is addQuestion
addQuestion(question:PracticeQuestion):any{
console.log("In QuestionManagementService: addQuestion: ",question);
this.bs.createQuestion(question).subscribe((res)=>{
console.log("add practice question - response is ",res);//I EXPECT THESE PRINTS TO SHOW BUT THEY DON'T
let ev = <HttpEvent<any>>(res);
if(ev.type === HttpEventType.Response) {
console.log('response from server: returning body '+ev.body);
let isResponseStructureOK: boolean = this.helper.validateServerResponseStructure(ev.body);
if (isResponseStructureOK) {
let response:ServerResponseAPI = ev.body;
console.log("received response from server: " + response.result);
this.addQuestionSubject.next(new Result(response.result,response['additional-info']));
} else {
console.log("received incorrect response structure from server: ", ev.body);
this.addQuestionSubject.next(new Result('error','Invalid response structure from server'));
}
}
else {
console.log("not response. ignoring");
}
},
(error:ServerResponseAPI)=>{
console.log("got error from the Observable: ",error);
let errorMessage:string = this.helper.userFriendlyErrorMessage(error);
this.addQuestionSubject.next(new Result('error',errorMessage));//TODOM - need to standardise errors
},
()=>{ //observable complete
console.log("observable completed")
});
}
As I am doing unit testing of addQuestion, I thought that I can mock the createQuestion method of the BackendService. The spec I have written so far is the following but I don't think it is correct as I don't see any prints on the console when the mocked response of createQuestion is received.
fit('should add a question',()=>{
let backendService = TestBed.get(WebToBackendInterfaceService);
let questionService = TestBed.get(QuestionManagementService);
let question = new PracticeQuestion(...);
const responseData = { result: 'success', ['additional-info']: 'question added successfully' };
let httpResponseEvent:HttpResponse<any> = new HttpResponse<any>({body:responseData});
//mock response of WebToBackendInterfaceService
spyOn(backendService,'createQuestion').and.returnValue(new Observable(()=>{
httpResponseEvent;
}));
questionService.addQuestion$.subscribe((res:Result)=>{
console.log('received response from Question Services',res);
expect(res).toBeTruthy();
let validResponse:boolean = ((res.result === 'success') || (res.result === 'initial')) ;
expect(validResponse).toEqual(true);
});
questionService.addQuestion(question);
expect(backendService.createQuestion).toHaveBeenCalled();
});

Answer 1

The issue was that I was creating an Observable but I didn't push the value to an Observer using next. The correct implementation is (code snippet)
spyOn(backendService,'createQuestion').and.returnValue(new Observable((subscriber)=>{ //subscriber or observer
subscriber.next(httpResponseEvent)}
));
When creating an Observable, the Observable's constructor in Rxjs takes a subscribe function as argument. The definition of subscribe function is
(observer)=>{
/*logic of calculating values which Observer should produce and emit then using observer.next(value)*/
}
reference - http://reactivex.io/rxjs/class/es6/Observable.js~Observable.html
With the above code, the this.bs.createQuestion(question) returns a mock Observable whose subscribe function is
(subscriber)=>{ //subscribe function
subscriber.next(httpResponseEvent)
}
The above subscribe function will be called whenever any Observer subscribes to the Observable. So in code .subscribe((res)=>{...}, I subscribe to the mock Observable and now the subscribe function will emit the dummy value, httpResponseEvent to my code. For those new to Observables, the subscribe function takes an Observer as argument. An Observer is an object which has 3 methods next, error and complete. Note that the subscribe function takes such an object
(res)=>{... },
(error:ServerResponseAPI)=>{
...
},
()=>{ //observable complete
...
})